1. Field of the Invention
The present invention relates to a solid-state image pickup device having photoelectric conversion elements (sensors) which are arranged two-dimensionally, such as a CCD solid-state image pickup device, and a method of driving the solid-state image pickup device.
2. Description of the Related Art
In order to enhance the dynamic range in an image sensor using a CCD solid-state image pickup device (hereinafter referred to as “CCD image sensor”), there have been proposed various methods of picking up images with photoelectric conversion elements which are different in sensitivity and then composing these pickup images into an image.
According to a first method of these methods, incident light is optically split to plural optical axes which are different in transmittance, and then the light beams thus split are detected by plural CCD image sensors arranged on the respective optical axes (see Japanese Laid-open Patent Application Nos. Hei-8-223491, Hei-7-254965, Hei-7-254966, Hei-8-340486 and Hei-10-69011, and U.S. Pat. No. 5,801,773).
According to a second method, an exposure time is divided into plural sub exposure times, plural images are respectively picked up at a different time and for a different sub exposure time by using one CCD image sensor, and then the plural images thus achieved are composed with one another (see Japanese Laid-open Patent Application Nos. Hei-8-331461 and Hei-7254965, U.S. Pat. Nos. 5,420,635 and 5,455,621, Japanese Laid-open Patent Application No. Hei-6-141229, U.S. Pat. No. 5,801,773, U.S. Pat. No. 5,638,118 and U.S. Pat. No. 5,309,243).
According to a third method, an image is picked up by using one CCD image sensor while the photoelectric conversion elements of the CCD image sensor are made different in sensitivity, and then signals detected by the plural photoelectric conversion elements different in sensitivity are composed with one another (see U.S. Pat. No. 5,789,737, Japanese Laid-open Patent Application No. Sho-59-217358, U.S. Pat. No. 5,420,635). In these prior art publications, it has been proposed that filters different in transmittance are covered on the respective photoelectric conversion elements as a method of varying the sensitivity of the photoelectric conversion elements in the CCD image sensor.
The first method needs plural CCD image sensors and a complicated optical system for splitting light, and thus has a problem that the manufacturing cost and the scale of the apparatus are increased.
According to the second method, information (image) detected with different sensitivity is achieved at a different time and for a different sub exposure time, so that it is difficult to accurately pick up the images of a moving subject and the resolution to a moving subject (hereinafter referred to as “dynamic resolution”) cannot be enhanced.
Further, the third method can solve the problems of the first and second methods, that is, both the problem that the apparatus is complicated and the problem that the dynamic resolution cannot be enhanced. However, in the third method, the filters different in transmittance must be covered on the photoelectric conversion elements to perform sensitivity control, and thus the sensitivity of each photoelectric conversion element is fixed in the manufacturing process of the CCD image sensor, so that the sensitivity cannot be variably controlled. Therefore, it is difficult to variably control the enlarging rate of the dynamic range in accordance with the condition.
In order to avoid the problem that it is difficult to variably control the enlargement rate of the dynamic range in accordance with the condition because the sensitivity of each photoelectric conversion element is fixed, Japanese Laid-open Patent Application No. Hei-9-191099 discloses a technique in which signals of a selected column are read out after a first accumulation time, then an electrical shutter is actuated, and then signals of columns other than the above selected column are read out after a subsequent second accumulation time, thereby enlarging the dynamic range.
However, as in the case of the method of covering the photoelectric conversion elements with the filters, the third method has a problem that the degree of freedom for the design of a spatial pattern of sensitivity is low, and for example it has a problem that signals having plural kinds of exposure times such as three or more kinds cannot be achieved in the vertical direction.
Further, since the first accumulation time and the second accumulation time are timely separated from each other by the charge sweep-out operation of the electronic shutter, the image pickup timing is different between a column from which charges are read out for the first accumulation time and a column from which charges are read out for the second accumulation time. Therefore, the dynamic resolution cannot be enhanced like the second method.
Still further, in order to promote the multi-pixel design or the compact design for the CCD solid-state image pickup device used for the CCD image sensor described above, the unit cell of each pixel is required to be designed in a more minute structure (that is, the microstructure of the unit cell is required to be further enhanced).